This invention relates to an automatic sewing machine for carrying out sewing operations according to stitch instructions which are read out or extracted in a predetermined sequence from a memory and include stitch position data designating a relative position between a needle and a workpiece, and more particularly to the drive motor speed control of the machine in the sewing operation.
It has been conventional practice in this type of sewing machines that a permissible drive speed, i.e., the maximum permissible drive speed of the drive motor is determined according to the kind of sewing operation specified to an individual machine. In other words, a machine which is performs sewing operations including a fairly large relative positional movement between the needle and the workpiece is alloted from the outset a comparatively low maximum permissible speed corresponding to the large relative movement allowed. This maximum permissible speed naturally regulates many other sewing operations which the automatic sewing machine can perform. A specific speed control for the drive motor for the automatic sewing machine is designated or determined according to the above-mentioned predetermined maximum permissible speed allotted thereto.
In this sort of automatic sewing machines, even when a sewing operation can be carried with comparatively small relative positional movements, the drive motor was in fact obliged to be limited to a maximum permissible speed of a comparatively low level irrespective of its capability of driving at a higher speed from the standpoint of the machine's construction and the performance of the drive motor. There thus arose a problem of using the machine at a performance level much lower than the true possible performance level when the sewing operation included only a smaller relative movement amounts between the needle and the workpiece, which degraded the efficiency of the sewing operation uselessly. In order to eliminate this disadvantage, U.S. Pat. No. 4,108,093 discloses a means for of installing a speed control circuit capable of regulating the speed of the drive motor so as to raise it up to the maximum permissible speed in accordance with a sewing operation of various types of stitches including the least relative movement amounts, and letting a memory store predetermined maximum permissible speeds in accordance with each sewing operation mode. In this cited reference, selection and change of the maximum permissible speed in accordance with the selection of each stitch pattern become possible, because various maximum permissible speed data can be stored permanently or fixed according to the modes of the stitch patterns used.
This prior art was still not free from some disadvantages. In the event of embroidery being done in an industrial sewing machine, wherein the number of stitches exceeded 1,000 and the relative movements between the needle and the workpiece varied in many ways, the maximum permissible speed was obliged to be predetermined at a comparatively low level speed corresponded to the largest relative movement anticipated. This meant that a large scale complex embroidery pattern must be formed at a certain low level speed which is predetermined, leading to a deterioration of the sewing operation efficiency.